When you are flying, one or both hands will be on the control yoke. The other may have to be on the throttle some of the time. Either way, this does not leave you many spare hands to use to measure an angle on your map with a protractor.
Over the years, therefore, pilots have had to find other ways to calculate angles quickly and easily. One of these is called the “1 in 60 rule”.
Imagine that you have a line on a piece of paper exactly 60 mm long:
Now raise a perpendicular at one end, exactly 10 mm high, and join them with a hypotenuse:
This will create an angle, z, in the above diagram. If the adjacent is 60 cm long and the opposite is 10 cm long, then the angle z will be 10°.
Similarly, if the adjacent is 60 cm long and the opposite is 5 cm high, z will be 5°. If the opposite is 8 cm high, z will be 8°, and so on – up to a maximum of about 20 degrees, when the theory starts to break down.